1. Field of the Invention
This invention relates generally to a method providing an adhesive tack coat for use between pavement layers, such as between layers of asphaltic compositions or between layers of asphaltic materials and other paving or base materials.
2. Description of the Background
In constructing pavement structures with one or more asphaltic layers, it is often necessary to lay each asphaltic layer down on a preceding pavement layer in a predetermined sequence, with a bonding material between the layers. The bonding material, which also may be referred to as a tack coat, acts to provide a degree of adhesion or bonding between the layers, and in some instances, may fuse the layers together. The tack coat also acts to reduce slippage and sliding of the layers relative to other layers in the pavement structure during use or due to wear and weathering of the pavement structure.
Insufficient adhesion between a new layer of pavement and an existing base course, a previously laid pavement layer, or a prepared pavement surface can cause pavement separation and cracking during construction of the structure, as well as subsequent failures and premature deterioration of the pavement structure and/or surface. Such conditions often require costly repairs, can cause damage to vehicles traveling on the surface, and may cause dangerous traffic conditions threatening damage to property and injury to vehicle passengers.
Pavement and road surfaces also require regular maintenance to repair normal wear and tear of the roadway due to vehicular or pedestrian traffic, the effect of weather and environmental conditions, and other factors. In some applications, all or a portion of the traffic bearing surface may be removed through grinding, stripping, or other means, and a new pavement layer is applied to the prepared structure. In other applications, a new pavement layer is placed down on top of the existing pavement structure to provide a new or renewed traffic-bearing surface. To prepare such preexisting pavement structures for application of an additional, new pavement layer, a tack coat often is applied to the preexisting pavement surface to encourage the adhesion and/or fusion of the new pavement material with the preexisting surface and/or structure.
In many instances, the bonding and/or slippage resistance provided by a tack coat is important during the construction of the pavement structure, as well as in the completed structure. As the pavement is constructed, it is important to avoid dislocation of all or portions of the substrate layers as each successive layer is applied to form the structure. Such damage can be significant at the edges of a roadway, which often are not compacted as well as in the middle of the roadway. In many applications, construction vehicles, or other vehicles are permitted to travel over the partially constructed structure, which also may exert stress on the layers that have been applied to form the roadway causing slippage or shifting of the layers.
A tack coat is a thin bituminous liquid asphalt, emulsion, or cutback layer applied between pavement layers to promote bonding. Adequate bonding between layers and especially between the existing road surface and an overlay is critical in order for the completed pavement structure to behave as a single unit and provide adequate strength. If adjacent layers do not bond to one another, they essentially behave as multiple independent thin layers—none of which are designed to accommodate the anticipated traffic-imposed bending stresses. Inadequate bonding between layers can result in delamination (debonding) followed by longitudinal cracking, potholes, and other distresses such as rutting that greatly reduce pavement life. Insufficient adhesion between the base course and roadway pavement material frequently is the cause of pavement separation and cracking during installation, and subsequent failures and premature deterioration of the pavement surface. Such failures may cause dangerous traffic conditions that require costly repairs.
The tack coat itself is typically composed of asphalt by-products and is sticky, an attribute that makes it ideal for use as an adhesive between layers of asphalt. Without the presence of the tack coat, a new layer of asphalt laid upon an existing road would deteriorate much more quickly, often creating potholes and various conditions that decrease the safety of the road.
A tack or bonding coat also may be applied to a base or substrate layer in advance of the application of the next pavement layer. In the meantime, construction or other vehicles are expected to travel over the tack coat without significantly damaging or reducing the effectiveness of the coating. Such damage often occurs, however, when the tack or bonding coating is picked up on the tires or tracks of vehicles traveling over the coated surface. Where this occurs, the asphalt compositions often are tracked onto other pavement surfaces causing disruption to the surrounding area. This tracking also reduces the effectiveness of the tack coat by displacing a portion of the intended volume from the area awaiting a new pavement layer.
Moreover, the tendency of a sticky tack coat to adhere to the tires and wheels of vehicles passing over the surfaces frequently results in the loss of the coatings on entire sections of the base course. This loss of coating substantially interferes with the ability of the tack coat to effectively prime the surface of the base course.
The tack coats typically used in the construction of asphaltic pavement structures are asphalt-containing compositions that are prepared as a liquid, flowable, fluid so that they can be effectively and efficiently applied and spread over a prepared base course or pavement surface. Such asphalt bonding compositions can be liquefied in several ways, such as by heating above their melting temperatures, dissolution into solvents or volatile oils to form cutback asphalt compositions, and by emulsification with water. In the case of cutback asphalt compositions, the cutback becomes “cured” as solvents evaporate leaving the desired asphalt composition.
The pavement surface receiving the tack coat should be clean and dry to promote maximum bonding. Emulsified tack coat materials may be applied to cool and/or damp pavement; however, the length of time needed for the set to occur may increase. Since existing and milled pavements can be quite dirty and dusty, their surfaces should be cleaned off by sweeping or washing before any tack coat is placed, otherwise the tack coat material may bond to the dirt and dust rather than the adjacent pavement layers. This can result in excessive tracking of the tack coat material. Construction vehicles and equipment pick up the tack-dirt mixture on their tires and leave the existing roadway with little or no tack coat in the wheelpaths. Slippage cracking and delamination are distresses typically seen when cleanliness is lacking.
To provide suitable bonding, tack coatings must provide an adhesive, tacky surface after the composition, emulsion, or coating is cured to bond and/or fuse the pavement or pavement and base layers. The asphalt materials available for use in tack coats are commercially available in different standard grades, with different properties, based on their viscosity, solids content, chemical composition, and other characteristics. One grading measure is the penetration or “pen” value.
The pen value is based on penetration testing where the relative hardness of the asphalt composition is determined at a predetermined temperature, typically about 77° F. (25° C.). One test uses methods such as those described in ASTM D5 (ASTM International) and AASHTO T49 (American Association of State Highway and Transportation Officials). This test measures the distance in dmm (tenths of a millimeter) that a standard needle, under a standard loading, will penetrate a sample in a given time under known temperature conditions. The resulting distance is often referred to as the penetration or “pen” value.
Another common measure of asphalt compositions is the Ring and Ball Softening Point. This softening point test using methods, such as those described in ASTM D36 and/or AASHTO T53, measures the temperature at which an asphalt composition becomes soft and flowable. As used herein, pen values and softening point values are with reference to the above-identified ASTM and AASHTO methods or their equivalents.
Although not formally defined, typically hard (i.e. high melting point) asphalt compositions have pen values of about 40 dmm or less, with softening points greater than about 140° F. (60° C.). Such compositions are often referred to as hard pen or low pen asphalt compositions. Asphalt compositions with pen values between about 40 dmm to about 100 dmm, and with softening points between about 118° F. (48° C.) and about 140° F. (60° C.), are typically referred to as medium, mid, or moderate pen asphalt compositions. Asphalt compositions with pen values greater than 100 dmm and with softening points less than 118° F. (48° C.) are typically referred to as soft or high pen asphalt compositions.
The asphalt compositions often used to form asphalt emulsions for the purposes of forming tack coats typically are soft or medium pen asphalt products. They typically have pen values greater than 40 dmm and softening points less than 140° F. (60° C.). When applied as a tack coat, emulsions using such soft or medium pen asphalt compositions typically provide a relatively sticky, tacky surface under ambient temperature conditions in pavement construction environments. As a result, such tack coats experience significant tracking problems, and may not provide a durable bond between paving surfaces, particularly at elevated temperatures.
Hard pen asphalt compositions (i.e., penetration value of less than about 40 dmm and softening points greater than about 140° F. (60° C.)) have not been used as tack coats in pavement construction due to their high melting points, their limited flowability and very low surface tackiness at ambient temperatures for pavement construction. For example, hard pen asphalt compositions often must be heated to temperatures as high as 400° F. (204° C.) before they become sufficiently flowable to be applied by spreading or spraying, and will have little, if any, surface tackiness when cooled to ambient temperatures for pavement construction. It is normally impractical to maintain hard pen asphalts at such high temperatures for application as a tack coat.
Hard pen asphalt compositions, in addition, are significantly more difficult to emulsify than mid to high (i.e., medium to soft) pen asphalt compositions. Because of their high melting temperatures, hard pen asphalt compositions typically must be heated to relatively high temperatures prior to emulsification, thus often resulting in undesirable emulsion temperatures above the boiling point of water (212° F. (100° C.)). The use of such high temperatures can interfere with the emulsification process, often is impractical in preparing most asphalt containing emulsions, and frequently produces emulsions with undesirable particulate contents and other defects. Additionally, hard pen asphalt emulsions often become unstable during storage and, therefore, must be used shortly after they are prepared. This also renders such products impractical as a tack coat in pavement construction.
Hard pen asphalt compositions and emulsions also have been used in applications where a high melting point is desirable. For example, hard pen asphalt compositions have been used in roofing materials and certain types of asphalt containing sealing compositions. Hard pen asphalt emulsions also have been mixed with clays and sands for use as a seal coating for the exposed surfaces of parking lots and the like.
As a result, cutback asphalts, asphalt emulsions using mid to soft pen asphalts, and mid to soft pen asphalts alone are commonly employed as tack coats for bonding asphaltic pavement materials to preexisting pavements, intermediate pavement courses, and base courses in new construction. Cutback asphalt tack coats typically included asphalt residues in excess of 60% by weight of the total product combined with solvents such as naphthas, kerosenes, oils or other such products to maintain the asphalt compositions in a liquid, flowable condition. The conventional asphalt emulsions typically included products with asphalt residues in a range of from about 25% to 70% or greater by weight of the total product.
A number of difficulties, however, limit the utility of cutback asphalts and conventional asphalt emulsions made with moderate or soft pen asphalt compositions. Some such products can require a relatively long cure time (as long as 2 to 7 days or more) before traffic is permitted to pass over the treated area. Where the next pavement layer cannot be laid down immediately, the cured tack coat remains exposed for extended periods and remains tacky and sticky. Accordingly, vehicle traffic over the treated surfaces often must be shut down until another layer of pavement is applied, and if traffic is allowed to pass, the asphalt tack coatings frequently adheres to the tires and tracks of vehicles traveling over the coated surfaces. This frequently results in lifting and damage to the tack coat and undesirable tracking of the coating to other surfaces via the vehicle tires and tracks.
Damage to the tack coating can substantially interfere with the ability of the coating to properly bond and/or fuse the pavement layers or base together. As a result, the pavement layers may slip or separate with the passage of traffic over the structure and time. This type of damage also may permit water penetration into the pavement surface, which can result in further deterioration due to freeze-thaw cycles or similar stresses. As a result, multiple applications of the tack coat may be required to obtain the desired adhesion between the pavement layers at a significant loss of efficiency and increase in cost. Furthermore, where the coatings are tracked to other surfaces by vehicles, extensive cleanup may be required adding to the expense of a project.
The use of conventional asphalts for tack coats, in addition, often requires equipment adapted to maintain those products at an elevated temperature throughout the application procedure. Such equipment is often expensive to operate and difficult to maintain, which increases the cost and reduces the efficiency of the asphalt tack coats. In addition, as it relates to cutback asphalt products specifically, they contain volatile solvents that can damage the environment and are released into the atmosphere during the curing process. These volatile solvents in asphalt cutbacks further present safety issues during their storage and application. As a result, cutback asphalt products are, or could become, prohibited from general usage in many states.